This hurt my brain

@Andy_P
I'm guessing this is done through changes in color gradients, and how fast the colors move through the circle. It's quite beautiful and intriguing.

@alyx You think it’s the speed of the spinning and how they line up with each other and not just the arrows? @Andy_P

@epic @Andy_P
It's not the arrows. I tried putting my thumb over the arrows, and the effect is still the same.

@alyx The simple answer is there’s no optical illusion at all. The illusion is you’re trying to tell your brain there’s no change, but they’re really moving and squashing the ‘circles’. 😀 @Andy_P

@epic @Andy_P
They're not moving. Put your mouse cursor as a reference point on the edges of the circles.

@epic @Andy_P
Something else that might influence the effect, is the thin lines that border the inside/outside of the circles.

@epic @Andy_P
Look closely at this frame. Notice how the inside and outside lines are differently colored between the left and right circles. They are basically mirror opposites.